Band-pass selector



April 1939- H. A. WHEELER 2,156,137

' BAND-PASS SELECTOR Filed Oct. 16, 1937 A L a 2|. 21, L3

FIG.2.

- VOLTAGE 0 +5 FREQUENCY DIFFERENCE.

(KILOOYOLES) INVENTOR HAROLD A.WHEELER FREQUENCY DIFFERENCE. (KILOCYCLES) ATTORNEY Patented Apr. 25, 1939 UNITED STATES PATENT OFFICE BAND-PASS snano'ron Harold A. Wheeler, Great Neck, N. Y. assignor to Hazeltine Corporation, a corporation of Delaware Application October 16, 1937, Serial No. 169,360

10 Claims. (CL 17844) This invention relates to band-pass selectors reference is had to the following description having a high degree of selectivity against intertaken in connection with the accompanying ference of frequencies near the desired frequency drawing, and its scope will be pointed out in the band and, while it is of general application, it is appended claims.

particularly suitable for use in the intermediate- Referring now to the drawing, Figs. 1 and '5 5 frequency channel of modulated-carrier signal are circuit diagrams of band-pass selectors inreceivers of the superheterodyne type. eluding different embodiments of the invention;

If the selector circuits of the intermediate-freand Figs. 2, 3, and 4 are diagrams illustrating cerquency channel of a superheterodyne receiver are tain of the-operating characteristics of the selecnot highly selective against interference of fretor circuit of Fig. l. 10. quencies near the desired band, a signal on a Referring now more particularly to Fig. 1 of channel adjacent to the channel of the desired the drawing, there is shown a selector circuit signal is likely to cause objectionable interference having input terminals I10, H- and output terresulting in unsatisfactory reproduction. To minalsl2, I3. There is provided a vacuum tube eliminate such interference, trap circuits have |4-,-which may be an amplifier tube in the interbeen used in the signal channel of the receiver mediate-frequency channel of a superheterodyne tuned l0 kilocycles above and below the mean receiver, having input electrodes coupled to terresonant frequency of the channel frequency. minals Ill, H and outputelectrodes coupled toa Because of inherent resistance in the tuned cirfirst tuned circuit C1L1 of the selector system.

cults, such trap circuits are inherently incapable There is also provided a second tunedcircuit com- 20:

of completely attenuating the interfering signal. pris o de C2 d inductance ae It is an object of the present invention to productance L2 physically comprising two inducvide a band-pass selector circuit having a high tances 2L2 coupled in parallel and one of the degree of selectivity against interference on freinductances' 2L2 being inductively coupled to the quencies near the desired band. inductance L1 of the first tuned circuit. A third 25.

It is another object of the invention to provide tuned circuit C3La is provided having its ina band-pass selector having infinite attenuation ductance L3 inductive y p to One 0fv th at the limiting frequencies of the'band without inductances 2L2 of the second tuned circuit. .A the use of any tuned circuits resonant outside cathode-biasing resistor I5 is provided for-tube the band. It. The parallel circuit CaLs is connected, in

In accordance with the invention, a band-pass series with resistance IE, to the output terminals selector system is provided including three or l3 0f the y The Output terminals more coupled tuned circuits, one of which is in- I3 are connected to the input circuit. of a vacuum cluded in the input circuit and another of which tube I6 which also may be an intermediate. is includedin the output circuit of the system. freq amplifying tube the detector tube Means are provided for deriving a voltage which of a superheterodyne receiver. Suitable. operatbears a predetermined relationship to, and is in mg potentials are provided for tubes l4 and. phase with, the input current of the first tuned in a conventional manner. The operation of the circuit of the selector and for combining such selector just described may best.be understood voltage with the voltage of said output tuned cirby re e 0 S- and cuit substantially in phase at the mean resonant F g- :3 Vector diagram indicating: the relafrequency of the system. As the frequency of tionship beteen the voltages across the resistor the input to the selector is varied, thephase of l5.and across the tuned circuit CaLa. The voltthe voltage across the output tuned circuit varies age across resistor i5 is represented by the refso that, at the limiting frequencies of the desired erencevector II, the phaseof which does not vary a band, this voltage is in phase opposition to the withthefrequency of the signal input to the input current of the first tuned circuit of the terminals 10, ot the system. .Vector |8 represelector. The system is balanced so that, .at sents the voltage acrossv the tuned circuit C3L3. these limiting frequencies, the. first voltage. exwhich-voltage varies in phase and'magnitude with m actly balances out the voltage across the output the frequency of the signalinput to terminals. tuned circuit of the selector, whereby infiiteat- 0, The System i ediuste S l' at at the tenuation is procured at these frequencies. mean resonant frequency of the selector, the For a better understanding of the invention, vectors I! and i8 are in phase. The diagram. I9 together -with-otherand further. objects thereof, represents-the. locus 0f the. vector l8 as the irequency of the signal input to the system is varied above and below the mean resonant frequency of the selector. Thus, it will be seen that, at two frequencies, one above and one below the mean resonant frequency of the selector, the voltage across the tuned circuit C3L3 is exactly in phase opposition to the voltage represented by vector i! which is the voltage across resistor 15. The constants of the circuit are so chosen that, at these frequencies, the vectors l1 and I8 are equal in magnitude.

Thus it will be seen that the vectors l1 and I8 are in phase at the means resonant frequency of the circuit and, inasmuch as the tuned circuit C3L3 and resistor [5 are in series across the output terminals I2, [3, the sum of these voltages appears across the output terminals l2, l3. As the frequency applied to the selector is varied from the mean resonant frequency of the system, vector 18 becomes displaced in phase with respect to vector I7 and also decreases in magnitude, until a frequency is reached at which these vectors are equal and opposite and infinite attenuation is obtained in the system. For frequencies further removed from the mean resonant frequency than these frequencies, there is a Slight output from the system, the voltage approaching that of the voltage across resistor l5. This is illustrated in the curves shown in Fig. 3, wherein the points at which infinite attenuation occurs have been chosen as 10 kilocycles above and below the mean resonant frequency of the system, as this is the frequency at which interfering signals are most troublesome in a superheterodyne receiver. The output voltage of the selector of Fig. 1 is represented by curve 20 of Fig. 3, while curve 2| represents the output voltage which would be obtained from the system without the addition of the constant voltage represented by vector l1 comprising the voltage drop across resistor l5.

Fig. 4 illustrates the output voltage frequency characteristics which may be obtained by the well-known expedient of increasing the coupling of the tuned circuits of the selector or by symmetrically detuning two of the circuits of the selector. These expedients are fully explained in a paper entitled High fidelity receivers with expanding selectors, by Harold A. Wheeler and J. Kelly Johonson, published in the Proceedings of the Institute of Radio Engineers for June, 1935. Thus, curve 22 represents the output voltage frequency characteristic of the selector incorporating the invention, while curve 23 illustrates the output characteristic which would be obtained if the resistor [5 of the invention were not included in series with the tuned circuit CsLs.

It will be understood that each of the three tuned circuits of the selector is preferably tuned to the mean frequency of the band to be passed by the system. It will be further understood that, if the voltage drop across the resistor I5 is in phase with the signalcurrent input to the first tuned circuit C1L1, as shown in the embodiment of Fig. 1, there must be an even number of degree reactive couplings between the several tuned circuits of the selector. If an odd number of couplings is required for any reason, the additional coupling must be a resistance coupling (producing zero phase shift) or a vacuum-tube coupling (producing degree phase shift).

Fig. 5 illustrates another embodiment of the invention and differs only slightly from the em- ;bodiment shown in Fig. 1-, like elements being given the same reference numerals in the two figures. The arrangement of Fig. 5 differs primarily from that of Fig. 4 in the method of obtaining a voltage in phase with the reference voltage across tuned circuit C3L3 at the mean resonant frequency of the system. For deriving this voltage, there is provided a tuned circuit 25, 26 connected in the output circuit of tube l4 and coupled to circuit C1L1 through resistor 21. There is also provided an inductance 28 inductively coupled to inductance 26 and having one terminal connected to the output terminal l3. A portion of inductance 23 is connected in series with tuned circuit CsLa by means of a connection between tap 29 and the low potential terminal of circuit CsLa. The inherent input capacitance of tube I6 is represented by the condenser 3i! shown in dotted lines in Fig. 5. For neutralizing the effect of the capacitance of condenser 30, a voltage from the lower portion of inductance 28 is applied through neutralizing condenser 3| to the grid of tube l6.

While four tuned circuits are shown in Fig. 5, only the three corresponding to the tuned circuits of Fig. 1 contribute to the phase shifting inciden- I tal to the attenuating action. The first tuned circuit is merely used to derive the reference voltage and the relatively constant voltage corresponding to the drop across resistor [5 of Fig. 1 and to couple this voltage to the output terminals. Inductance 28 is closely coupled to the inductance 26 across which the constant voltage is developed.

Circuits 25, 26 and C1L1 may be moderately broadly tuned, while circuits Calm and Gale may be sharp and closely coupled or symmetrically detuned to provide a fiat-top over-all frequencyresponse characteristic in the manner described with reference to Fig. 1.

While there have been described what are at present considered to be the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A band-pass selector system for transmitting frequencies of a given frequency band, comprising means for deriving a first voltage variable in phase in accordance with the frequency of the input to the system, means for deriving a second voltage which varies less rapidly in phase than said first voltage with the frequency of the input a to said system, the constants of said selector being so proportioned that said two voltages are in phase at a frequency near the center of said band and opposite in phase at frequencies near each of the limiting frequencies of said band, and means for deriving an output voltage variable in accordance with the vectorial sum of said first and said second voltages.

2. A band-pass selector system for transmitting frequencies of a given frequency band, comprising means for deriving a first voltage variable in phase and magnitude in accordance with the frequency of the input to the system, means for deriving a second voltage which varies in phase and magnitude with the frequency of the input to said system less rapidly than said first voltage, the constants of said selector being so proportioned that said first voltage is in phase with said second voltage at the mean resonant frequency of said band and opposite in phase and equal in magnitude to'said second voltage at the limiting frequencies of said band, and means for deriving an output voltage variable in accordance with the vectorial sum of said first and said second voltages.

3. A band-pass selector system fortransmitting frequencies of a given frequency band, com-prising means including a network of coupled tuned circuits for deriving a first voltage variable in phase and magnitude in accordance with the frequency of the input to the system, means-for deriving a second voltage which varies in phase and magnitude with the frequency of the input to said system less rapidly than said first voltage, the constants of said system being so proportioned that said first voltage is in phase with said second voltage at the mean frequency of said band and opposite in phase and equal in magnitude to said second voltage at the limiting frequencies of said band, and means for deriving an output voltage variable in accordance with the vectorial sum of said first and said second voltages.

4. A band-pass selector system for transmitting frequencies of a given frequency band, comprising means including a network of coupled tuned circuits each tuned to the mean frequency of said band for deriving a first voltage variable in phase and magnitude in accordance with the frequency of the input to the system, means for deriving a second voltage which varies in phase and magnitude with the frequency of the input to said system less rapidly than said first voltage, the constants of said selectors being so proportioned that said first voltage is in phase with said second voltage at the mean frequency of said band and opposite in phase and equal in magnitude to said second voltage at the limiting frequencies of said band, and means for deriving an output voltage variable in accordance with the vectorial sum of said first and said second voltages.

5. A band-pass selector system for transmitting frequencies of a given frequency band, comprising means including an odd number of inductively coupled tuned circuits for deriving a first voltage variable in phase and magnitude in accordance with the frequency of the input to the system, means for deriving a second voltage which varies in phase and magnitude with the frequency of the input to said system less rapidly than said first voltage, the constants of said selector being so proportioned that said first voltage is in phase with said second voltage at the mean frequency of said band and opposite in phase and equal in magnitude to said second voltage at the limiting frequencies of said band, and means for deriving an output voltage variable in accordance with the vectorial sum of said first and said second voltages.

6. A band-pass selector system including input and output terminals for transmitting frequencies of a given frequency band, comprising a vacuum tube having input and output electrodes, said input electrodes being connected to said input terminals, means coupled to said output electrodes for deriving a first voltage variable in phase and magnitude in accordance with the frequency of the input to the system, additional means connected in circuit with said output electrodes for deriving a second voltage which varies in phase and magnitude with the frequency of the input to said system less rapidly than said first voltage, the constants of said selector being so proportioned that said first voltage is in phase with said second voltage at the mean frequency of said band and opposite inphase and equal in magnitude to said second voltage at the limiting frequencies of said band, and means for derivingan output voltage variable in accordance with the vectorial sum of said first and said second voltages.

7. A band-pass selector system including input and output terminals for transmitting frequencies over a'given'frequency band, comprising a'va'cuum tube having input and output electrodessaid input electrodesbeing connected to said input terminals,-a network of coupled tuned circuits coupled to said output electrodes for deriving a first voltage variable in phase and magnitude in accordance with the frequency of the input to the system, additional means connected in circuit with said output electrodes for deriving a second voltage which varies in phase and magnitude with the input to said system less rapidly than said first voltage, the constants of said selector being so proportioned that said first voltage is in phase with said second voltage at the mean frequency of said band and opposite in phase and equal in magnitude to said second voltage at the limiting frequencies of said band, and means for deriving an output voltage variable in accordance with the vectorial sum of said first and said second voltages.

8. A band-pass selector system including input and output terminals for transmitting frequencies of a given frequency band, comprising a vacuum tube having input and output electrodes, said input electrodes being connected to said input terminals, means coupled to said output electrodes for deriving a first voltage variable in phase and magnitude in accordance with the frequency of the input to the system, means comprising a resistor connected in circuit with the output electrodes of said tube for deriving a second voltage the phase and magnitude of which are substantially independent of the frequency of the input to said system, the constants of said selector being so proportioned that said first voltage is in phase with said second voltage at the mean frequency of said band and opposite in phase and equal in magnitude to said second voltage at the limiting frequencies of said band, and means for deriving an output voltage variable in accordance with the vectorial sum of said first and said second voltages.

9. A band-pass selector system including input terminals for transmitting frequencies of a given frequency band, comprising a vacuum tube having input and output electrodes, said input electrodes being coupled to said input terminals, a network of coupled tuned circuits coupled to said output electrodes for deriving a first voltage variable in phase and magnitude in accordance with the frequency of the input to said system, means comprising a tuned circuit coupled to said output electrodes and an inductance coupled to said tuned circuit for deriving a second voltage which varies in phase and magnitude with the frequency of the input to said system less rapidly than said first voltage, the constants of said selector being so proportioned that said first voltage is in phase with said second voltage at' the mean frequency of said band and opposite in phase and equal in magnitude to said second voltage at the limiting frequencies of said band, and means for deriving an output voltage variable in accordance with the vectorial sum of said first and said second voltages.

10. A hand-pass selector system including input and output terminals for transmitting frequencies of a given frequency band, comprising a network. of coupled tuned circuits coupled to said input terminals for deriving a first voltage variable in phase and magnitude in accordance with the frequency of the input to said system, means comprising a tuned circuit coupled to said input terminals and an inductance coupled to said tuned circuit for deriving a second voltage which varies in phase and magnitude with the frequency of the input to said system less rapidly than said first voltage, the constants of said selector being so proportioned that said first voltage is in phase with said second voltage at the mean frequency of said band and opposite in phase and equal in magnitude to said second voltage at the limiting frequencies of said band, means for deriving a voltage variable in accordance with the vectorial sum of said first and said second voltages and applying it between said output terminals, a vacuum tube having input electrodes coupled to said output terminals, said tube having an appreciable input capacitance, and means including said inductance for coupling a voltage into the input circuit of said tube for neutralizing the effect of said capacitance.

' HAROLD A. WHEELER. 

